The entrainment, generation and expression of circadian rhythms can be disrupted by social stress. In this project, a resident-intruder social conflict paradigm with hamsters will be used to study the relationship between circadian rhythms and social stress. Hamsters are spontaneously aggressive animals, and an animal that has been repeatedly defeated by an aggressive conspecific will eventually become defensive/submissive when exposed to a nonaggressive opponent. A resident animal that will not defend its home cage territory against an intruder will be categorized as a conditioned defeat (CD) animal. In a series of experiments, we will employ the CD paradigm to investigate potential pharmacological agents (diazepam and melatonin) that can influence circadian rhythms. Different lighting schedules, the timing of drug administration, and melatonin secretion will be investigated. The following aims will be addressed. Specific Aim 1 - we will determine if constant (24 hours) light or dark can have an influence on CD. Specific Aim 2 - we will determine if low dose of diazepam (1.0 mg/kg IP) during different lighting conditions can effectively attenuate CD. Specific Aim 3 - we will focus on melatonin functioning: 1. we will determine if exogenous administration of melatonin (20 mg/kg IP) can attenuate CD; 2. we will determine if CD can have an influence on endogenous levels of melatonin; and 3. we will explore if melatonin secretion is controlled by dopamine. This project will explore the underlying neural mechanisms that can affect circadian rhythms during social stress.